There are known conventional semiconductor devices made of silicon, such as a trench-type Si-MOSFET. Japanese Patent Laid-Open No. 06-132539 discloses a semiconductor device having a vertical insulated gate bipolar transistor that includes a semiconductor substrate of a first conductivity type, a first semiconductor layer of the first conductivity type of low impurity concentration formed on the principal surface of the semiconductor substrate, a second semiconductor layer of a second conductivity type formed on the upper surface of the first semiconductor layer, a third semiconductor layer of the first conductivity type formed on a part of a surface part of the second semiconductor layer, a gate oxide film formed on an inner wall surface of a gate trench, which has a substantially U-shaped cross section and extends from the surface of a central part of the third semiconductor layer through a part of the second semiconductor layer into the first semiconductor layer, a gate electrode formed to fill a groove on the gate oxide film, an insulating film formed to cover the gate electrode and an exposed surface of the second semiconductor layer, a gate wire that is formed on the insulating film and is in contact with the gate electrode, a source electrode that is in contact with the third semiconductor layer through a contact hole formed in the insulating film, and a drain electrode formed on the back surface of the semiconductor substrate. Japanese Patent Laid-Open No. 06-132539 discloses a structure in which the gate trench is formed in a ring-like shape.
A silicon carbide semiconductor device made of silicon carbide, such as a SiC-MOSFET, has a high breakdown voltage, so that the gate trench can be insufficient to prevent the electrical field from excessively concentrated in the gate oxide film during reverse biasing, and the gate oxide film can break.
To avoid this, a protection trench can be disposed around the gate trench to prevent an electrical field from being applied to the gate trench. However, such a protection trench can fail to prevent an electrical field from being locally excessively concentrated during reverse biasing because of the interval between sections of the protection trench or the shape of the protection trench in plan view (in the horizontal direction), for example. If an electrical field is locally excessively concentrated in the protection trench, the withstand voltage of the protection trench decreases, and a desired withstand voltage cannot be achieved. In addition, if there is a part that has a low withstand voltage, a problem arises in that the avalanche resistance decreases.